Line formation analysis in transmission geometry. Top: relative peak absorption for Hα (black), the MgIb line at ∼5183 Å (red), the Cal line at ∼6439 Å (blue), the Cair K line (green), the FeI line at ∼4071 Å (magenta), and the FeII line at ∼5169 Å (orange). Bottom: relative absorption in a 100 Å spectral bin centered at the MgI resonance line at ∼2850 Å (black), the MgII h&k resonance lines at ∼2800 Å (red), the FeI NUV absorption band at ∼2720 Å (blue), the FeII resonance line at ∼2600 Å (green), and the FeII NUV absorption band at ∼2370 Å (magenta) as a function of pressure. In both panels, the dashed line shows the TP profile (top x-axis). The line formation region corresponds to the pressure values in which the relative absorption ranges between zero and one.